Dave Knapp, Extension Master Gardener, Anoka County
Dahlias are among the most colorful of all flowers and come in small, medium, large and dinner-plate sizes. Dahlias can be started from seed but are easiest started from tubers, which are the fleshy tuberous root of a mature dahlia.
Choose a sunny location with six to eight hours of direct sunlight. Dahlias can be grown in part shade, but results may not be as spectacular. Lighter soils work best; if you have heavy soil, incorporate some peat moss or sand. Go easy adding compost as most compost is high in nitrogen. Too much nitrogen will result in lots of “plant” but few flowers.
Plant in spring when all chance of frost is gone and the ground temperature has reached 60 degrees; late May is typically a good time. Dig holes six to eight inches deep every twenty-four inches. Lay the tuber in the hole on its side with any sprout three inches below the surface.
Before covering the tuber, drive a stake into the hole to help support the plant as it grows. Driving the stake at this time will avoid damaging the tuber. Cover and tamp the soil. Do not water unless your soil is very dry, then only lightly. No watering should be necessary until sprouts appear above ground, then water lightly once a week until plants are well established.
Fertilizer may be applied after three to four weeks and again about August 1st. Dahlias are high users of nitrogen, but too much nitrogen will decrease flowers, resulting in tubers that do not winter well during storage.
Dahlias have some pests but a Sevin spray every 7-10 days when plants are 3-6 inches tall will control most of them. A systemic spray can also be used.
When plants are 15 inches tall, a terminal flower bud is set. Remove this bud and any others with it. Doing so will improve plant growth and develop strong stems to support later flowers. When buds reappear again and are ¼ inch diameter, they will be in groups of three or more. Leave the largest bud and remove the others next to it. This will encourage larger flowers and avoid stems breaking under the weight of the flower. This is especially necessary for dinner plate flowers to achieve their intended size.
Dahlias will flower until the first killing frost. Dig up tubers for storage 10-14 days after a killing frost, or November 15th, whichever comes first. Stalks can be cut to 6-8 inches during this time. When digging, gently lift the entire clump of tubers, wash off all dirt and allow to dry. Divide clumps at time of storage or in the spring. Each cut tuber should include an eye. Not all tubers will have an eye; those without eyes should be discarded, as they will not be viable. Store in cardboard boxes or crates lined with newspaper and suitable storage medium like wood chips, peat moss or sawdust. Store boxes in a cool dry area at 40-50 degrees - then think spring and enjoy dahlias galore.